1. Field of the Invention
This invention relates to the production of shale oil and, more particularly, to a method of in-situ retorting oil shale to recover the shale oil.
2. Description of the Prior Art
The oil in oil shale is present in the form of a solid material referred to as "kerogen". To recover oil from oil shale, the shale is usually retorted at a high temperature in kiln-type equipment at the surface whereby the kerogen is decomposed into a liquid product which can be further refined to produce gasoline and other fuels. Mining and transporting the shale to the surface have been an important part of the cost of recovery of oil from shale. Oil shale is frequently found in deposits at depths below the surface of the ground large enough to preclude use of strip mining methods. Sometimes the shale deposits are very thick, for example, up to 2,000 feet and are not suitable for mining by conventional underground mining methods. If room and pillar mining, such as is used in the mining of coal, should be employed, the pillars left to support the ceiling cause a large part of the shale in the deposit to be left in place when the mining is completed.
U.S. Pat. No. 3,466,094 of Haworth et al describes a mechanical mining operation in which the shale is blasted into an underlying chamber by explosives in shot holes extending from an exposed face of the chamber. The broken shale is transported to the surface for retorting. In the process described in U.S. Pat. No. 3,537,753 of Arendt some of the costs inherent in conventional mining methods are avoided by drilling shot holes from the ground surfaces through the overburden into the shale deposit, but as in the room and pillar mining method, a substantial part of the shale is left in the formation, and it is necessary to precede the shot hole drilling with some expensive conventional mining operations.
To avoid the high cost of mechanically mining shale, attempts have been made to retort the shale in situ. In the process described in U.S. Pat. No. 1,422,204 of Hoover et al. a plurality of boreholes are drilled into the shale deposit and explosives detonated in the borehole to form fractures providing communication between the boreholes. Steam or hot combustion gases are then circulated through the cracks from one borehole to another to heat the shale to a temperature high enough to cause decomposition of the kerogen and drive volatile carbonaceous products to the surface through some of the boreholes. Oil shale has a very low permeability and the fracturing caused by detonating explosives in a borehole will not provide communication from that borehole to an adjacent borehole adequate for effective retorting of the shale. Only the shale immediately adjacent to any fracture formed by the explosion is heated and decomposed by hot gases passing through the fracture. Unless the shale is broken into small particles to form a rubble, most of the shale is bypassed and is not decomposed by the hot gases. However, if the shale subjected to the blasting is confined, conventional commercial explosives will not form a rubble of small enough pieces to permit efficient retorting. If the shale is not confined and can move when subjected to the force of the explosive, as in the blasting referred to as relieved blasting, satisfactory rubblizing can be obtained.
U.S. Pat. Nos. 3,661,423 of Garrett and 3,316,020 of Bergstrom describe processes in which the shale is broken into rubble to permit in-situ retorting of the shale and intimate contact between the hot gases used in the retorting of the broken shale. The processes of U.S. Pat. Nos. 3,661,423 and 3,316,020 require substantial room and pillar type mining operations and the drilling of horizontal holes through the shale.
In U.S. Pat. No. 3,001,776 of Van Poollen a plurality of vertical cylindrical chambers filled with rubble for retorting are formed by drilling boreholes from the surface and detonating explosives in the boreholes. The vertical chambers are separated by columns of shale which are not retorted in the process. Consequently, a substantial part of the oil in the shale deposit cannot be recovered by the process described in the patent. U.S. Pat. No. 3,465,819 of Dixon describes a process of retorting in vertical cylinders in which a chimney is formed below the shale deposit by a nuclear explosive. Conventional explosives are used to break horizontal layers of shale from the deposit and drop the thus-broken shale into the chimney where the shale is retorted. Control of the size of the chimney and rubblizing produced by nuclear explosions is difficult. Moreover, substantial amounts of unretorted shale are left in the formation between chimneys.